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Heat pump delta T too low - 2c

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(@benseb)
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So we have an Ecodan 14kW heat pump. Had it a few years and it’s worked well but a bit expensive. 

whenevrr I’ve checked the stats the delta between flow and return has always been 5c on the dot. 

This year I’ve been balancing radiators and removing zones to make one big zone to improve efficiency. Which it has!

 

However I’ve noticed that now the Delta T is 2c most of the time. 3c max. 

Which would indicate not enough heat is leaving the rads / UFH?

What do I need to check/adjust?

 

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@benseb)
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B1AC69D2 5447 4002 917E 3EDD47470F03

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@derek-m)
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Posted by: @benseb

So we have an Ecodan 14kW heat pump. Had it a few years and it’s worked well but a bit expensive. 

whenevrr I’ve checked the stats the delta between flow and return has always been 5c on the dot. 

This year I’ve been balancing radiators and removing zones to make one big zone to improve efficiency. Which it has!

 

However I’ve noticed that now the Delta T is 2c most of the time. 3c max. 

Which would indicate not enough heat is leaving the rads / UFH?

What do I need to check/adjust?

 

Is your heat pump running in weather compensation mode and if so, what are the slope settings?

Are you getting much cycling?

The temperature readings may vary dependent upon the speed of the water pump and whether the water is fully mixed by the time it reaches the sensors. Obviously monitor what is happening, but if your system appears to be functioning correctly it may not be something of great concern.

 


   
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(@benseb)
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@derek-m Thanks Derek

 

Were in Weather Comp mode. 45c @-2,   25c @ 12c

This has been fine, but I have bumped it up +4 this week with it being windy as our house can be a bit leaky and it’s quite exposed. 

As far as cycling, I can’t see much evidence of this. Maybe 1 or 2 cycles per hour but mostly a steady flow temp. Heating has been actively running for 45% of the time in the last 7 days but we do have a log burner which tends to turn it off in the day and a 2c overnight setback so sounds about right?

 

I think our heat pump flow is a bit low (29l/s vs recommended 35-40) but not much we can do about that without a lot of upheaval. 

I was wondering if the flow on the secondary side was correct. No way to measure that. They just used the existing pipework and whacked a 40-70 Evosta DAB pump on it, on full whack! We also have an UFH zone coming off it too. I’ve no way really of measuring the flow and knowing If the pump is on too high? (We have a buffer so it’s not same flow rate as HP)

 

ben

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@derek-m)
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@benseb

Without knowing the full details of your system, including the heat loss calculations, it is difficult to provide advice.

Do you have any thermostats within your system, and if so, are they actively switching your secondary pump and heat pump on and off?

Running a wood burner will further complicate the control of the system. Is your wood burner on all day? Where is it located? Does it draw air from the room or does it have a separate ducted supply?

I am not surprised that you had to set your WC at +4, since LWT of 25C @ 12C ambient is quite low. LWT of 25C @ 20C would probably be more appropriate.

A buffer will further complicate your system even more. In an ideal World, the water flow rate going into the top of the buffer from your heat pump, will be matched by the flow rate of the warm water being drawn out of the top of the buffer and fed to the heat emitters. The return water flow rate from the heat emitters going back into the bottom section of the buffer, will be matched by the flow rate of the return water being drawn out of the bottom of the buffer and sent to the heat pump. We don't live in an ideal World.

More than likely there will be mixing of the water within the buffer, which could lead to cooler water being fed to the heat emitters and lower overall efficiency.

As far as temperature and flow measurements are concerned, it is unlikely that they will be totally accurate, and the accuracy could even vary under different operating conditions.

If you have a suitable thermometer, I would suggest measuring the temperature at the inlet and outlet pipes at your buffer and compare these with the ones provided by the controller. Note the changes with the heat pump running, and after it has been stopped for a period of time. Monitor the operation under varying weather conditions.


   
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(@alec-morrow)
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Balancing with weather compensation on gas boilers and heat pumps is about distributing heat evenly and appropriately through the system, not about achieving any particular delta t accross the rads.

 

So open the valves more to enable the system to stabilise as it was before!

 

 

 

 

 

 

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(@heacol)
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@benseb the Delta T should be between 5 and 10 Deg C. Try reduce the speed on one of the circulating pumps. This will increase the Delta T and further reduce your running cost. Do you have a buffer tank? If so see if you can pipe it out, this will further increase your performance.

Professional heat pump installer: Technical Director Ultimate Renewables Director at Heacol Ltd


   
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(@benseb)
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Topic starter  

Thanks all. 

I will do some measurements and also try opening some valves

 

i did try turning the pump down from 3 to 1 but the delta didn’t shift, still 2c

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@alec-morrow)
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What are people trying to achieve by widening the delta 2

 

the best approach is maximum flow and key the heat pump decide the rest

 

gas boilers are slightly different, but with weather compensation a delta t is fine especially at low temps

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(@derek-m)
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@benseb

I would not get too fixated on the DeltaT reading, which may not be correct anyway. As Alec has pointed out, concentrate more on how your system is performing. If you have a suitable thermometer then measure the temperatures around your buffer, and record them over a period of time under different operating conditions. Ensure that the thermometer is in good thermal contact with the pipework and suitably insulated from the surrounding air.

If you need any help in interpreting the results then please post them on the forum.


   
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(@scrchngwsl)
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The Heat Geek YouTube channel have released all their data on their own office's heat pump openly here: https://emoncms.org/heatgeek/graph . You can see that their DeltaT is typically less than 3 degrees C. Echos what the experts here are saying, but nice to have actual data from a real install to compare with.

ASHP: Mitsubishi Ecodan 8.5kW
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Battery: 8.2kWh


   
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(@heacol)
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@scrchngwsl The ideal Delta T for a heat pump is 8 Deg C. Any variance, either way, will reduce performance. The good units on the market will control this temperature gap by varying the compressor and pump speeds to achieve permit performance.

Professional heat pump installer: Technical Director Ultimate Renewables Director at Heacol Ltd


   
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